The present invention is directed to improvements in the utilization and disposal of waste lignocellulosic material generated by processing plants, and particularly of wood waste generated by the wood products industry.
In the wood products industry, huge quantities of wood waste material such as sawdust, shavings, chips and bark are generated by lumber and plywood mills. In other industries utilizing lignocellulosic raw materials, such as sugar cane processing, similar large quantities of waste lignocellulosic material such as bagasse are produced. Disposal of these waste materials in a useful way has primarily been either by using them as a fuel or as a raw material for particle board manufacture. As a fuel the waste materials present problems in that they normally have too high a moisture content for efficient burning and therefore must be dried first, which consumes substantial amounts of energy. For use of the waste materials in particle board manufacture, the energy-consuming drying requirement is even more of a problem than it is with respect to use of the material as a fuel, since a greater degree of drying is required to render the material suitable as a particle board furnish. Generally speaking, wood waste materials originally have moisture content in the range of 50% to 60% by weight and should be dried to 30% to 35% moisture content for use as a fuel and to approximately 9% moisture content for use as a particle board material.
A totally separate problem which also has long plagued the wood products industry is the high levels of pollutants in stack gas emissions from wood processing plants. The problem exists not only with respect to stack gas effluents from furnaces or boilers utilizing wood wastes as fuel (where the problem is especially severe due to the difficulty in burning wood waste completely), but also from the gaseous effluent of wood dryers as well. In the case of wood waste combustion emissions, the primary pollutants include fly ash and unburned hydrocarbons, the latter in large part being submicron sized particles which are especially difficult to remove. Salt particles, also present, are also very small in size and difficult to remove from the gaseous effluent from wood-burning furnaces.
In the case of wood dryer emissions, the primary pollutants are condensible hydrocarbons distilled from the wood. These present special problems because of their gummy nature when condensed, which causes them to adhere to whatever equipment is used to remove them from the stack gas effluent, requiring frequent cleaning of the equipment.
The dryers used in the past to reduce the moisture content of waste lignocellulosic materials for use as furnace fuels, such as those disclosed in Caughey U.S. Pat. No. 4,258,476 of Mar. 31, 1981, Boulet U.S. Pat. No. 4,114,289 of Sept. 19, 1978, and Williams U.S. Pat. No. 3,826,208 of July 30, 1974, have also released significant quantities of pollutants such as dust and condensible hydrocarbons. requiring filtration of the gaseous exhaust from such dryers. Additionally, U.S. Pat. Nos. 4,258,476, 4,114,289, and 3,826,208 have disclosed use of effluent hot gases to dry lignocellulosic materials for use as furnace fuels. Such methods and apparatus for drying have, however, resulted in freeing quantities of potentially pollutant dust, etc. from the materials being dried, necessitating use of exhaust stream cyclone separators, filters, and the like to limit air pollution.
The filtration of effluent gases through filter beds of finely divided granular particles of various compositions has long been practiced, as evidenced by such disclosures as those of Rigg U.S. Pat. No. 1,095,676 of May 5, 1914, Squires U.S. Pat. No. 3,296,755 of Jan. 10, 1967, Zenz U.S. Pat. No. 3,770,388 of Nov. 6, 1973, Zenz U.S. Pat. No. 3,800,508 of Apr. 2, 1974, and Squires U.S. Pat. No. 4,006,533 of Feb. 8, 1977. However such filter bed gas treatment processes have not taught how compatibly to maximize the filtration of pollutants, particularly of the submicron size, from the effluent gases while simultaneously causing significant drying of the filter material. This is because effective filtration and effective drying are basically incompatible due to the reliance of filtration upon high moisture content of the filter material to "wet" solid pollutants of small size and condense and wet volatile pollutants.
Moreover, the nongranular, generally flake-like nature of comminuted wood waste particles has been thought to render them unfit as a filter bed material because of the substantial resistance to the passage of gaseous effluents therethrough which the overlapping flake-like particles would cause, resulting in a substantial back pressure on the stack gases.
In addition, filter beds have not generally been considered appropriate for filtering stack gases containing gummy or sticky condensible hydrocarbon pollutants of the type generated from the drying of lumber and wood veneers because of the difficulty of attempting to clean the pollutant deposits off of the screens, louvers or vanes which contain the filter material in a filter bed.
Previous attempts to use comminuted lignocellulosic material as filter material have also met with problems of blockage of the perforations of a support membrane such as a woven screen material on the outlet side of the filter bed.
Apparatus and a method are needed, then, for utilizing lignocellulosic waste material for efficiently filtering pollutants including hydrocarbon distillates and submicron sized particles of salts, fly ash, and unburned hydrocarbon solids from a polluted stream of effluent gas, while simultaneously sufficiently drying the waste material to render it useful.